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R90/6 with dual plugs

I'm restoring a 1974 R90/6. It's had a couple of upgrades over the years before I became the owner. Ignition, rear shocks, and dual plugs. Somewhere along the way, it was tapped for dual plugs, but nothing else was done. No coil, etc. Right now the bike runs just fine with spark plugs in the bottom of each cylinder hooked up to nothing. Any kind of plug I can put in there and remove the bottom spark plugs. Don't think I'll ever consider finishing the dual plug thing. Seems to be a debate as to if it improves anything.

I'll bet if you offered to swap there is somebody out there that would like to have the dual plugged heads and would give you their unmodified heads in return. Some risk, I know, but an option to consider.

PPO of my '76 R75/6 dual plugged it, PO didn't like how it ran and went back to running single plugs. I continued with the single plugs until the Dyna III electronic ignition went flaky. I then decided to go back to points with an amplifier as if the amplifier dies, it can be pulled from the circuit and run as originally designed. During this period, there was no problem with the idle spark plugs acting as just a "plug" in the head. I see no reason not to continue as things are.

I later decided to try out dual plugging, as the expensive work had already been done, the cost being two new coils, fresh plugs, a second set of spark plug wires and a set of Ted Porter's ATU shims. Before adding the shims, starting was a bit rough, but now it starts more easily than it ever had for me. I now run regular gas without pinging.

My R90S (sold) had dual plugs when I got it, but the second set was not hooked up. It ran fine with points, but a bit hard to start and would ping from time to time. I just left the second plugs in as finding something to plug the hole would have been harder. I also found out that the modification could use two type of plugs, those with coarse or fine threads. My R90S had the coarse threads, my 78 Motorsport (also modified) had the fine threads. I sold the bike to a person that installed a Dyna III with new coils and he said the pinging went away and it started easier. I think the modification may make the bike run better, but if you are happy I would just leave well enough alone. My next airhead (soon to own a 77 R100S) will not be modified.

A dead plug in the lower hole will eventually get crudded up with carbon. Which is not good. A special shorty bolt of proper thread size should be in that lower hole if you run one plug. My R90 had a dead plug in the bottom for about 5K. when removed a couple years back it was heavily carboned up.

When an old dual system was installed the bike woke up. There's a reason somebody thought about adding a lower plug to the old hemi-head design, and it's called flame front or flame propagation or gas that is no longer available. Tons of commentary and advice has been written on the topic which should be read.

The lower hole many times was bored at 12mm because of less metal between a 14mm hole and the valve seats. Two cents only IMO.

I had a dual plug setup on my '77 R100S. It was a very hard starter. I converted it back to a single plug setup, and frankly it ran awesome, and started flawlessly with little effort. I just left the plugs in the lower sockets.

I hated how hard the bike started, so set the valves, balanced the carbs, made sure the plugs were all good, etc. Then I took it to my airhead mechanic. His first recommendation was to put it back to a single plug per head.

Definitive Source of Information on Dual Plugging

I still have my paper copy of this from when i dual-plugged my '78 R100RS.

Dual plugging was done to compensate for lower octane fuels that were becoming more prevalent in the early 80's, for engines designed to run on higher octane gas. Primary effect was to eliminate pinging, or pre-detonation. A side benefit was slightly improved performance overall. If everything in the d-p set-up was done correctly, starting should have improved as well, or at least it would not have gotten worse. Those above who said their bike was hard to start... something was amiss in your set-up (maybe a cracked or defective coil, incorrect timing, etc.)

Timing

You have to reset the timing at or close to TDC or the engine will be fighting itself and be hard to start.With one plug the timing is set before TDC to allow the flame front to develop.With dual plugs this happens much quicker at TDC or close to it.

If everything in the d-p set-up was done correctly, starting should have improved as well, or at least it would not have gotten worse. Those above who said their bike was hard to start... something was amiss in your set-up (maybe a cracked or defective coil, incorrect timing, etc.)

I agree 100%. My 78 R100/7 has been double-plugged since 1985 and running a Dyna ignition. I have a real problem believing people see improved starting by going back to points and single plugs. Dual plugging made a big improvement to my bike and I will never go back! My timing is at TDC and another poster mentions the necessity of retarding the timing as starting the combustion process in two places gets the fire going quicker.

My 1971 R75/5 (dual plugged) usually starts in less than a second and is very uncritical where I have the choke (but these are with Mukuni carbs). I have my ignition timing set at 1 degree retarded from TDC at idle. That is a full ten degrees retarded from the stock single plug heads, which is nine degrees advanced for stock.

But if the Dyna coils are even the slightest bit weak there are all kinds of weird starting problems, at least with mine. Perhaps the Dyna coils should be changed every five years or so. I've notice the first system of weak coils is the choke is very touchy for when starting and takes MUCH longer to start. But with stock Bings that might be different, I don't know.

More voltage is needed for the two plugs in series on each side and it is more critical to have the higher voltage.

When I hear about trouble starting with dual plugs, my first guess is weak Dyna coils. Timing left as stock would be my second guess. My R75/5 starts instantly and has a lot of guts, but I have new Dyna coils. I would say the bike performs a lot better than when the bike was new.

I do believe the dual plugs help a lot as long as they have very high voltage on the plugs. With a weak spark, you would be better off without the dual plugs. No pinging heard, not , ever even accelerating up hills on the cheapest gasoline.

Isn't it important to consider how the plugs and coils are connected? DonTom mentioned that it takes more voltage for plugs in series on one side. It does take more energy for the spark to jump under compression than not under compression. Therefore, shouldn't one coil power the top plugs and the other coil power the lower plugs? The advantage would be that the arrangement is just like a single-plug setup where one coil is dealing with only one plug under compression at one time. Another advantage would be that if one coil fails, the system reverts to a single plug setup by default.

What Kurt recommends is mentioned in several places and makes sense if you have a single pickup. But, if you have a dual pickup it is almost impossible to get the plugs to fire at the same time.

My Motorsport had dual pickups and I never could get them to fire exactly at the same time. I verified this with two timing lights connected to the same cylinder, one on each plug. You could see two timing marks when I reversed the wires. Thus, the one that fired first in the cylinder would do all the work since it would ignite the gas before the other one fired. You would be back to a single plug configuration since the second plug was doing nothing. I tried readjusting the pickups several times to get them the same, never could.

What Kurt recommends is mentioned in several places and makes sense if you have a single pickup. But, if you have a dual pickup it is almost impossible to get the plugs to fire at the same time.

My Motorsport had dual pickups and I never could get them to fire exactly at the same time. I verified this with two timing lights connected to the same cylinder, one on each plug. You could see two timing marks when I reversed the wires. Thus, the one that fired first in the cylinder would do all the work since it would ignite the gas before the other one fired. You would be back to a single plug configuration since the second plug was doing nothing. I tried readjusting the pickups several times to get them the same, never could.

Wayne

Yep. I have two pickups so I never tried to change, but I have wondered about doing such. But with a bike that runs so well, I could think of any reason to fool around with the configuration. But I can't say enough how important it is to prevent starting problems by having the very maximum high voltage possible to all four of the plugs.

So yeah, there are starting issues with dual plugs, but my first question would be how old are the Dyna coils and are the plug wires okay? It should start better and faster than stock, but that will only happen with good Dyna coils. Mine could not start better, but my Dyna coils are only a year old. Before these last ones were replaced, I had some serious starting issues too. What a difference with the new coils!

Even if the plugs fire a micro second sooner or later, than the other, the main development is that the fuel mixture not yet burned lurking around the perimeter of the chamber, waiting to pre ignite, will get burned before pinging. Maybe later in the powerstroke, nonetheless, burned. IMHO two cents. Agree with others as to ease of starting with four plugs and a noticeable gain in performance minus pinking. R90 timed at TDC, with ancient Accel amp. And 28 year old Andrews coils.

Even if the plugs fire a micro second sooner or later, than the other, the main development is that the fuel mixture not yet burned lurking around the perimeter of the chamber, waiting to pre ignite, will get burned before pinging. Maybe later in the powerstroke, nonetheless, burned. IMHO two cents. Agree with others as to ease of starting with four plugs and a noticeable gain in performance minus pinking. R90 timed at TDC, with ancient Accel amp. And 28 year old Andrews coils.

8ninety8, I'm not picking on you, just using your post as a segway to my commments. As a life-long hands-on engineer it bothers me when I see mis-information being disseminated. The four plugs in a double-plugged engine all fire at the same time because the ignition system has two dual-output coils that are connected in series. When the points open, or in an electronic system a magnet sweeps past a trigger coil, the magnetic fields in both coils collapse simultaneously and all four plugs fire. The plugs are not in series. There is no issue of the first plug hogging the energy leading to a weak spark at the second. If anyone sees one plug firing before another I can only conclude there is an issue with your equipment or methodology.
Also every time this comes up people start talking about pre-ignition and pinging like they were the same thing. At least once I read a poster commenting on "pre-detonation". Pre-ignition and detonation, (pinging), are two distinctly different things. Pre-ignition occurs when the fuel charge starts burning before the spark happens, pre-ignition. It is usually caused by a hot piece of carbon inside the combustion chamber, an over-heated spark plug, or a hot valve, likely a leaking exhaust valve. Detonation is caused when the rising temperature inside the combustion chamber raises the fuel end mixture to its auto-ignition point and it explodes spontaneously hammering the top of the piston, (ping!). Usually this is an issue of low octane fuel because octane is after all a measure of a fuel's ability to resist detonation.